Spatial light modulator is an electro-optical element that is electronically controlled to alter either the path or the intensity of an incident light to obtain desired output light signal. One type of the spatial light modulator is an interferometric spatial light modulator built by microelectromechanical systems (MEMS). Such spatial light modulator has two closely placed surfaces. The distance between these two surfaces may be controlled so that the system selectively absorbs or reflects light guided by the principles of optical interference, in regarding to a pre-defined way, or in regarding to commands from a driving system. Further more, with the help of MEMS technology, an array of such spatial light modulators can be integrated onto a small chip or a glass substrate, therefore realize applications in display field.
Microelectromechanical systems (MEMS) include micro mechanical elements, actuators, and electronics. The micro mechanical elements and the actuators may be created using depositing, etching, and other microfabrication processes.
MEMS-built interferometric light modulators are known from U.S. Pat. No. 5,835,255, issued to Miles, U.S. Pat. No. 6,674,562, issued to Miles for interferometric modulators of radiation. In these references, a method is provided making a spatial light modulator and a display apparatus of an array of such spatial light modulators, where the interferometric cavity gap changes spontaneously with external applied electronic signal and therefore modulates the incident light intensity. Such a typical spatial light modulator has two layers to define a cavity. One layer serves as a mirror, and one or both of the layers serve as electrodes. One layer is semitransparent to allow light pass through, to reach the surface of the second layer and get reflected there. One layer is movable relatively to the other through two or more positions upon different electrical signals applied, which causes the cavity to operate interferometrically in these positions. In one position, the movable layer is at relaxation and the spatial light modulator is at bright state; in another position, the movable layer is at actuated state and the spatial light modulator is at dark state. In these references, a type of display apparatus is also claimed, which includes an array of such interferometric modulators formed integrally on a substrate, preferably transparent, and its control circuitry connected to the array for controlling each modulator independently to switch between its bright and dark states. The display apparatus uses passive multiplexing addressing.
It would be desirable to have a compact spatial light modulator with active addressing and high contrast, and to have a display apparatus with an array of such integrated compact spatial light modulators. Such display apparatus, for example but not limited to, microdisplay, would be good for displaying large amount of information.
As easily understood by those skilled in the art, specular reflection is the nature characteristic of interferometric spatial light modulator and the display made of such modulators. This characteristic makes such a display especially favorable for projection display applications.
On the other hand, advance of integrated circuit technology has provided the foundation for active addressing of silicon-based microdisplays. Examples of such microdisplays are shown in U.S. Pat. Nos. 6,023,309 and 6,052,165, where microdisplays of liquid crystal on silicon (LCOS) are made. It is easily understood by those skilled in the art that such active addressing schemes can be adapted to the microdisplays made of spatial light modulators built on silicon substrate.